Amplifiers have increasingly been employed in electrical devices which have the requirement of being as low in noise as possible placed upon them. For example, in receivers for wireless systems, such as mobile phones, a low-noise amplifier, or LNA, is employed at the input of the receivers. In this context, it is the task of the LNA to amplify a very weak input signal while adding as little additional noise as possible to the signal, so that a signal having as small a noise portion as possible will result at the output of the LNA.
If a high signal is present at an input of the amplifier or LNA, this may cause subsequent amplifier stages to become saturated. In this manner, a signal amplified by the amplifier stages may then be distorted such that, for example, in a system of several amplifier stages, a bit error rate, or BER, will be increased on the receiver side, or a signal quality will be reduced.
FIG. 6 shows a potential low-noise amplifier. The low-noise amplifier is configured as a potential common-gate LNA 11, or CG LNA 11.
The common-gate LNA 11, or common-gate terminal LNA, in this context comprises a field-effect transistor 13, a load impedance 15, an operating-point voltage source 17, an operating-point constant current source 19, an input voltage source 21, and an impedance 23 of the input voltage source.
The drain terminal of the field-effect transistor 13 is connected to a supply voltage terminal via the load impedance 15. A gate terminal, or control terminal, of the field-effect transistor 13 is coupled to a ground terminal via the operating-point voltage source 17. A bulk terminal, or substrate terminal, of the field-effect transistor 13 is also connected to a ground terminal. The operating-point constant current source 19 is connected, at a first terminal, to the source terminal of the field-effect transistor 13 and, at a second terminal, to a ground terminal. A first terminal of the input voltage source 21 is connected in an electrically conducting manner to the source terminal of the field-effect transistor 13 via the impedance 23 of the input voltage source. At the same time, a second terminal of the input voltage source 21 is connected to a ground terminal.
The operating-point constant current source 19 specifies an operating point of the potential common-gate LNA 11, the DC component which flows through the load impedance 15 approximating the current of the operating-point constant current source 19. The operating-point voltage source 17 adjusts a potential difference between the source terminal and the gate terminal of the field-effect transistor 13, so that the constant current from the operating-point constant current source 19 may flow through the field-effect transistor 13.
Upon application of an AC voltage signal from the input voltage source 21, a potential difference between the gate terminal of the field-effect transistor 13 and the source terminal of the field-effect transistor 13 will vary, as a result of which the current flowing through the load impedance 15 will vary.
The amplification of the common-gate LNA 11, or common-gate terminal LNA, is determined, in a first approximation, by the load resistance, or the load impedance, 15 Zload and by a transconductance gm of the LNA MOSFET, or metal oxide semiconductor field-effect transistor, which depends on the specified operating point of the field-effect transistor 13. In addition, the transconductance gm specifies the input impedance of the potential common-gate LNA 11.
In order to avoid reflections, the input impedance is adapted to the impedance of the preceding stage, such as a source of an antenna, a filter, etc. This adaptation is referred to as matching, or input matching.
In order to reduce the amplification of the common-gate LNA 11, the load impedance 15 commonly has an additional resistor, such as a MOSFET, connected in parallel therewith. If the load impedance 15 consists, in this context, of a resonant circuit, such as a load coil at a drain terminal of the MOSFET or field-effect transistor 13, a parasitic capacitance of the field-effect transistor 13, and an input capacitance of the follower stage, the resistor connected in parallel will reduce the quality of the circuit and thus increase the bandwidth of the circuit. This increase in bandwidth is frequently undesired and disadvantageous.
What is unfavorable or disadvantageous about the potential common-gate LNA 11 shown in FIG. 6 is that the input impedance of the potential common-gate LNA 11 will change as a result of a change in the DC operating-point current, or direct current operating-point current, which is mainly supplied by the constant current source 19. This is why the DC operating-point current frequently is not changed so as to adjust the transconductance gm and, thus, the amplification. Instead, an additional resistor is advantageously connected in parallel, as was already mentioned above.
In this context, an input impedance Zin is roughly reciprocal to the transconductance gm. Here, the following connection applies:
                              Z                      i            ⁢                                                  ⁢            n                          ≈                  1                      g            m                                              (        1        )            
In this context, the amplification of the circuit 11 may be determined from the formula below.
                                          G            v                    ≈                                                                u                out                                            u                s                                                                =                                            g              m                        ·                          Z              load                                            1            +                                          g                m                            ·                              Z                S                                                                        (        2        )            
In the above formula, a factor Gv is an amplification of the common-gate LNA 11, a variable uout is an amplitude of an AC voltage component of the output signal, us is an amplitude of an AC voltage of the input source 21, Zload is an impedance value of the load impedance 15, and ZS is an impedance value of the impedance 23 of the input voltage source.
In an input matching, wherein the input impedance Zin of the LNA circuit equals the impedance of the input voltage source ZS, the following connection will then apply:
                              G          v                ≈                                            g              m                        ·                          Z              load                                2                                    (        3        )            
FIG. 7 shows a potential common-source LNA 24, or CS LNA. In the following, elements which are identical or have identical actions will be designated with the same reference numerals. In addition, in the potential common-source LNA 24 shown in FIG. 7, only the differences relative to the common-gate LNA 11 shown in FIG. 6 will be discussed. Unlike the potential common-gate LNA 11 shown in FIG. 6, the potential common-source LNA 24 shown in FIG. 7 comprises a BIAS resistor 27, a source inductance 29, a gate inductance 29a, and a coupling capacitor 31.
Unlike the common-gate LNA 11 shown in FIG. 6, in the potential common-source LNA 24 shown in FIG. 7, the source terminal of the field-effect transistor 13 is connected to a ground terminal via the source inductance 29. In addition, a voltage source 25 for specifying the operating point is connected to the gate terminal of the field-effect transistor 13 via the gate inductance 29a by means of the BIAS resistor 27. At the same time, in the potential common-source LNA 24 shown in FIG. 7, the input voltage source 21 is connected, via the impedance 23 of the input voltage source, to an input terminal 33 of the common-source LNA 24. The input terminal 33 of the common-source LNA 24 is coupled to the gate terminal of the field-effect transistor 13 via a coupling capacitor 31 and the gate inductance 29a. 
The source inductance 29 and the gate inductance 29a serve to eliminate the complex components of a gate/source capacitance in the input impedance Zin at a predetermined frequency, so that the input impedance will only comprise active components. This connection will be explained in more detail below. The voltage source 25 for specifying the operating point specifies a potential difference between the gate terminal of the field-effect transistor 13 and the source terminal of the field-effect transistor 13, and adjusts the operating point of the field-effect transistor 13 and, thus, an amplification of the potential common-source LNA 24. The coupling capacitor 31 serves to filter out potential DC signal components, or DC components.
Depending on the AC voltage signal of the input voltage source 21, a potential difference between the gate terminal of the field-effect transistor 13 and the source terminal of the field-effect transistor 13 will change, and thus the current flowing through the field-effect transistor 13 between the source terminal and the drain terminal will also change. This change in the current flowing through the field-effect transistor 13 leads to a change in the current flowing through the load impedance 15, and thus to a change in the output voltage. The AC voltage signal from the input voltage source 21 is thus amplified by the potential common-source LNA 24.
The input impedance of the potential common-source LNA 24 may be calculated from the formula:
                              Z                      i            ⁢                                                  ⁢            n                          =                              r                          g              ,              NQS                                +                                                    g                m                            ·                              L                s                                                    C              gs                                +                      1                          sC              gs                                +                      s            ⁡                          [                                                L                  s                                +                                  L                  g                                            ]                                                          (        4        )            
In the above formula, Zin represents a value of the input impedance, and rg,NQS represents a so-called non-quasi static resistance. A variable Ls represents a value of the source inductance 29, whereas a variable Cgs represents the gate/source capacitance. A Laplace variable s may be equated with a complex angular frequency jω or j2πf. In addition, a variable Lg represents the gate inductance.
If a condition of a so-called input matching is met, i.e. if the value of the input impedance Zin equals the value of the impedance 23 of the input voltage source, the following connection will apply:
                              Z                      i            ⁢                                                  ⁢            n                          =                              Z            S                    =                                    r                              g                ,                NQS                                      +                                                            g                  m                                ·                                  L                  s                                                            C                gs                                                                        (        5        )            
A precondition for the above connection in the input matching is that the magnitudes of the terms 1/sCgs and s[Ls+Lg] be equal, so that no reactance components will occur in the input impedance Zin.
The connection
                              r                      g            ,            NQS                          =                  1                      κ            ·                          g              m                                                          (        6        )            applies to the so-called non-quasi static resistance. In this context, K is a so-called Elmore constant, which here has the value of 5. The amplification of the circuit will then result from the following connection:
                              G          v                =                                                                        u                out                                            u                s                                                          =                                    1              2                        ·                                          g                m                                            ω                ·                                  C                  gs                                                      ·                                          Z                load                                            Z                S                                                                        (        7        )            
In the above formula, a variable ω symbolizes the magnitude of the angular frequency 2πf.
The low-noise amplifier circuits shown in FIG. 6 and FIG. 7 have the task of amplifying very weak input signals while adding as low a noise as possible to the signal amplified. If a high signal is present at the input of the LNA, this may cause subsequent stages to become saturated. This may lead to an increase in the bit error rate, or BER, of the system. With very large input signals, the amplification of the LNA will therefore be reduced.
In this context, in the potential common-gate LNA 11, the specification of the operating point, and thus the amplification, could be changed by a change in the DC operating-point current. Thus, in the event that a large signal which is provided by a preceding stage such as a filter is present at the input of the LNA, the amplification may be reduced, so that the subsequent stages will not become saturated. However, this would entail a change in the input impedance. This change in the input impedance, in turn, would result in that for the input voltage source 21 with the impedance 23 of the input voltage source, and for the LNA circuit, or the potential common-gate LNA 11, the condition of input matching is no longer met.
In this context, reflections of the signal provided by the input voltage source 21 could then occur at the potential common-gate LNA 11.